1. Field of the Invention
The present invention relates to the field of electrochemical cells, and in particular electrochemical cells having both high energy and power density.
2. Description of the Related Art
The lithium rechargeable battery is an attractive technology due to its comparatively high energy density, low potential for environmental and safety hazard, and relatively low associated materials and processing costs. The lithium battery is charged by applying a voltage between the battery's electrodes, which causes lithium ions and electrons to be withdrawn from lithium hosts at the battery's cathode. Lithium ions flow from the cathode to the battery's anode through an electrolyte to be reduced at the anode, the overall process requiring energy. Upon discharge, the reverse occurs; lithium ions and electrons are allowed to re-enter lithium hosts at the cathode while lithium is oxidized to lithium ions at the anode, an energetically favorable process that drives electrons through an external circuit, thereby supplying electrical power to a device to which the battery is connected.
Currently known cathode storage compounds such as LiCoO2 and LiMn2O4 when used with currently known anodes such as lithium metal or carbon have working voltages between 3 and 4V. For many applications a high voltage and low weight are desirable for the cathode as this leads to high specific energy. For example, for electrical vehicle applications the energy-to-weight ratio of the battery determines the ultimate driving distance between recharging. The power density of the battery is equally important for many applications. For example, in a hybrid electric vehicle the power density has great influence on the rate of acceleration, or the rate at which regenerated braking energy can be recaptured by the battery system. Thus for many applications both a high energy density and a high power density are necessary. In addition, it is desirable to have lithium storage materials that are low in cost and nontoxic to the environment. It is also desirable to have storage materials that when combined with other components of a battery system such as liquid electrolyte remain safe. It is also desirable to have storage materials that are stable and have a long operating life over a wide range of temperatures.
Cathodes in state-of-the-art rechargeable lithium batteries contain lithium ion host materials, electronically conductive particles to electronically connect the lithium ion hosts to a current collector (i.e., a battery terminal), a binder, and a lithium-conducting liquid electrolyte. The lithium ion host particles typically are particles of lithium intercalation compounds, and the electronically conductive particles are typically made of a substance such as a high surface area carbon black or graphite. Commonly used cathode storage materials such as LiCoO2, LiNiO2, and LiMn2O4 and their derivatives suffer from the disadvantages of high cost, toxicity, instability at elevated temperatures, or instability in the charged state.
Anodes for rechargeable lithium-ion batteries typically contain a lithium ion host material such as graphite, electronically conductive particles to electronically connect the lithium ion hosts to a current collector (i.e., a battery terminal), a binder, and lithium conducting liquid electrolyte. Carbon-based anodes have the disadvantage of a relatively low volumetric energy density due to their low density of about 2 g/cm3.
Lithium transition metal polyanion compounds are of interest for electrochemical applications, including as storage cathodes for rechargeable lithium batteries, due to their potentially high energy density, low raw materials cost, environmental friendliness, and safety. Such polyanion compounds include LixMXO4 of the olivine structure, LixM2(XO4)3 of the NASICON structure type, VOPO4, LiFe(P2O7) and Fe4(P2O7)3, and derivative structures that have additional interstitial metal ions, symmetry-changing displacements, or minor changes in the connectivity of polyhedra. Polyanion compounds typically contain compact tetrahedral “anion” structural units (XO4)n−, (X=P, S, As, Mo, W, Si, Al, and B) with significant covalent bonding character, which are joined in a manner producing other sites such as oxygen octahedra or tetrahedral that are occupied by other metal ions.
It was previously accepted that this class of compounds has a fundamental limitation of low electronic conductivity, desirable for solid electrolytes but limiting in applications as ion-storage or fuel cell electrodes. Indeed, the published literature contains many references by those skilled in the art to the insulating nature of these compounds, and the corresponding limitations on their utility as battery storage materials. These references include “Approaching Theoretical Capacity of LiFePO4 at Room Temperature at High Rates,” H. Huang, S.-C. Yin and L. F. Nazar, Electrochem. Sol. St. Lett., 4[10] A170-A172 (2001); J. Gaubicher, T. Le Mercier, Y. Chabre, J. Angenault, and M. Quarton, “Li/β-VOPO4: A New 4 V System for Lithium Batteries,” J. Electrochem. Soc., 146[12] 4375-4379 (1999); and “Issues And Challenges Facing Rechargeable Lithium Batteries,” J.-M. Tarascon and M. Armand, Nature, 414, 359-367 (2001).
The gravimetric energy densities of several of this class of compounds are attractive when compared with previously used cathode-active materials. However, due to crystal densities that are typically lower than those of cathodes such as ordered rocksalt structure compounds like LiCoO2, LiNiO2, and orthorhombic LiMnO2, or spinel structure compounds like LiMn2O4, or their derivatives, alkali metal transition metal polyanion compounds typically have lower volumetric energy densities. Electrochemical cells including storage batteries that utilize these polyanion compounds therefore also tend to have lower volumetric energy densities.
Thus, it has heretofore not been possible to obtain an electrochemical cell using lithium-ion intercalating compounds having high power density, safety, and long life while also having high energy density comparable or superior to conventional lithium ion cells.